This invention relates generally to single-frequency solid state lasers. More particularly, it relates to lasers in which frequency tuning with temperature occurs without mode hops.
Single-frequency solid state lasers find many applications in coherent communications, laser radars, and as pump lasers for nonlinear optical frequency conversion, among other applications. The frequency of operation of the laser depends on two important features. First, the solid state gain medium has a characteristic frequency response, generating light in a particular continuous frequency range by stimulated emission in response to pumping, often by laser diodes. Second, the laser cavity has a particular total optical path length (product of physical path length and index of refraction) that determines the allowed discrete wavelengths of axial modesxe2x80x94only an integral or half-integral number of wavelengths is allowed. The gain frequency range is a continuous curve, often a Lorentzian distribution, within which many axial modes are allowed. FIG. 1 illustrates a typical gain envelope supporting many possible axial modes. The spacing of axial modes, known as the xe2x80x9cfree spectral rangexe2x80x9d is inversely proportional to cavity length. In general, only the axial modes near the peak gain wavelength can support lasing.
Various techniques have been developed to ensure single-frequency laser operation. The simplest is to use a very small cavity, which provides a large axial mode spacing, and therefore limits the number of modes that will fit under the gain envelope. In general, these monolithic resonators, in which the cavity is defined by the physical boundaries of the crystal itself, exhibit many problems, including poor pump absorption and exclusion of intracavity tuning elements. They also exhibit significant spatial hole burning, a phenomenon caused by overlap of two portions (or directions) of the beam path in the gain medium. The standing-wave patterns resulting from interference of the beams, when only one axial mode oscillates, gives rise to incomplete gain saturation at the nodes. These low-saturation regions exhibit higher gain than the rest of the laser material and preferentially overlap with the standing wave pattern for adjacent axial modes of the resonator, leading to oscillation of two or more modes at once. In response to these problems, birefringent tuning filters and coupled cavity designs are used to provide frequency-dependent loss for mode discrimination over a wide gain bandwidth. These solutions tend to be very lossy and complex. Twisted mode resonators induce circular polarization, eliminating spatial hole burning. Ring cavities eliminate counter-propagating beams altogether using frequency-selective elements to force unidirectional oscillation.
Even when single frequency operation is attainable using one of the above methods, the problem of axial mode hopping is still present. This phenomenon can be understood by considering the graph of FIG. 1. The shape and horizontal position of the gain curve 10 is a function of temperature. In particular, the peak gain "ugr"g has a particular peak gain frequency tuning rate with respect to temperature (d"ugr"g/dT), which is usually negative (i.e. increased temperature leads to a decreased frequency). Each axial mode 12 also has a particular tuning rate with respect to temperature, which depends on the optical path length, and therefore on the thermal expansion coefficient and thermo-optic coefficient (refractive index rate of change with temperature) of the cavity elements. In a monolithic laser, in which the entire cavity is defined by the crystal with polished edges, the cavity optical path length is determined solely by properties of the lasing material. These two temperature responses (i.e., peak gain and axial mode) are completely physically independent. In general, the two tuning rates are unequal, with the axial mode tuning rate usually greater for solid state monolithic lasers. As the temperature of the gain medium is increased, the axial mode frequency "ugr"m nearest to the peak decreases more quickly than the peak gain frequency "ugr"g, and eventually a different axial mode 12 at a higher frequency will be closer to the peak gain. As a result, the laser will xe2x80x9chopxe2x80x9d over to that higher frequency mode.
This behavior is illustrated in FIG. 2, a graph of axial mode photon energy versus crystal temperature for a monolithic Nd:YAG laser, a common solid state laser. The peak gain temperature shift is given by the overall slope of the curve 20, while the piece-wise slope is the tuning rate of the axial mode. The discontinuities in the curve are the mode hops, which occur approximately every 4-5 xc2x0 C. for a 20 mm long cavity. Obviously, these mode hops are unacceptable for stable laser operation. Since the mode hops illustrated in the curve of FIG. 2 are reproducible, it is possible to operate a laser at a temperature away from the discontinuities. Often, however, it is either impossible or undesirable to maintain the crystal temperature within such a small temperature range.
The problem of eliminating mode hops has not been solved satisfactorily in solid state lasers. However, it has been addressed with some success in other types of lasers with very different operating conditions. A laser with an actively stabilized etalon for frequency selection is disclosed in U.S. Pat. No. 5,144,632, issued to Thonn. A parameter indicative of the laser""s output frequency, either output power or current supplied, is monitored, and the etalon temperature is varied to maintain operation at a desired frequency. This solution has two significant drawbacks that make it unsuitable: it requires a complicated feedback system for frequency stabilization, and it does not really address the problem of mode hops. Rather, it maintains a fixed frequency, keeping operation away from regions in which mode hops occur.
The problem of mode hopping has also been addressed in semiconductor lasers, which have much different operating conditions than solid state lasers. A waveguide DBR laser containing a semiconductor gain element and a waveguide grating functioning as a resonant cavity end reflector is disclosed in U.S. Pat. No. 5,870,417, issued to Verdiell et al. Various methods of suppressing mode hops are addressed in this device. Minimizing the length of the optical cavity increases the free spectral range, making it less likely for mode hops to occur within a relatively narrow temperature range. The device is also designed with particular lengths and thermal conductivities of substrate material so that the optical path length remains constant with an increase in temperature. As the gain element increases in size with increasing temperature, the end reflector moves closer toward the gain element to maintain a constant optical path length. As a result, the mode frequency remains relatively constant and therefore near the peak gain frequency, which has a tuning rate with a lower magnitude. A third solution is to maintain the entire device at a constant temperature, so that the operation frequency is fixed, and the tuning rates become irrelevant. The device can also be maintained at a temperature between mode hops, but not necessarily at a single exact temperature. Note that these solutions prevent the laser from having a useful tuning range. Other methods include changing the refractive index of the gain medium by current pumping in response to temperature changes of the device. These methods require a very uniform and constant temperature throughout the device. Either the temperature is controlled to be within a very narrow range, or the temperature is assumed to remain within a narrow range. Maintaining a constant optical path length ensures that the axial modes do not shift with temperature. Mode hopping is prevented only if the peak gain frequency also remains virtually constant. As discussed below, maintaining a narrow temperature is unsuitable for solid state laser operation.
The problem of mode hopping in semiconductor lasers is also addressed in U.S. Pat. No. 4,583,227, issued to Kirkby et al. The device is constructed with a semiconductor gain element and an external reflector located at a known distance from the gain element and mounted on a support that shifts with temperature. The support structure is chosen to have a particular size and thermal expansion coefficient so that its response to temperature fluctuations is predetermined. Four different embodiments are disclosed: two in which the semiconductor face opposite the reflector is partially reflective, and two in which the face has an anti-reflective coating. In the latter two embodiments, the support structure moves with temperature either to maintain a constant composite cavity optical path length, as was done in the device of Verdiell et al., or to provide a composite cavity thermal expansion coefficient that is equal to the peak gain wavelength expansion coefficient. In the first of these options, the device must still be operated in a narrow temperature range, so that the peak gain wavelength remains virtually constant. In the second case, which does not require such a narrow temperature range, a thermal expansion coefficient that is not physically realizable is required. The required coefficient is much larger than that of existing materials, and is achieved only by a complicated structure of layered supports of varying thermal expansion coefficients. This complicated solution derived by Kirkby et al. is not applicable to solid state lasers. In semiconductor lasers, the axial mode tuning rate is usually greater than the peak gain tuning rate, while in solid state lasers, the peak gain tuning rate is greater than the axial mode tuning rate. This puts completely different design requirements on solutions for avoiding mode hops in the two types of lasers. The method Kirkby et al. teach for matching the two frequency tuning rates is based on proper choice of the expansion coefficient of the substrate. The entire cavity must therefore expand at a known rate.
In fact, none of these various semiconductor laser solutions can be applied to solid state lasers to achieve the desired mode-hope-free operation. Most of the solutions deal only with stabilizing the composite cavity path length and therefore axial mode, and are only suitable for a narrow temperature range in which the peak gain remains relatively constant. More importantly, however, semiconductor lasers and solid state lasers have significantly different operating conditions, and even the final solution of Kirbkby et al., equating the cavity expansion to the peak gain wavelength expansion, is unsuitable. This method requires that the entire laser cavity, including gain medium, supports, and external reflector, be at a uniform temperature, even as the temperature changes. While this requirement is achievable in low power, single frequency semiconductor lasers, it is virtually impossible to achieve in solid state lasers. Solid state laser materials generate significant waste heat, on the order of Watts or tens of Watts, while semiconductor materials generate only milli-Watts of waste heat. Waste heat is localized within the solid state gain material, and it is virtually impossible to keep the rest of the laser cavity at the same temperature. Waste heat generation also fluctuates throughout the gain material itself, causing fluctuations in the temperature of the gain material but not in the temperature of the remainder of the cavity. These temperature fluctuations are often so large that it is not feasible to keep the laser operating between mode hops. Furthermore, semiconductor lasers have cavity lengths on the order of 300 xcexcm, providing for large free spectral ranges of many GHz, while solid state lasers have much smaller free spectral ranges, down to hundreds of MHz. These small free spectral ranges require much tighter axial mode control, not achievable with the solutions listed above.
There is still a need for a solid state laser in which mode hops are prevented over a large temperature tuning range and without need for complicated temperature control and feedback systems.
Accordingly, it is a primary object of the present invention to provide a solid state laser that is tunable over a wide temperature and frequency range without mode hops.
It is a further object of the invention to provide a simple design for a mode-hop-free solid state laser that can be applied to a wide range of solid state gain materials.
It is an additional object of the invention to provide a mode-hop-free laser design that can be applied to all types of single-frequency solid state lasers, including standing wave, ring, and non-planar ring oscillators.
It is another object of the present invention to provide a mode-hop free laser in which preventing mode hops is inherent in the laser design, and requires no active feedback control mechanism.
It is an additional object of the present invention to provide a mode-hop free laser that does not require a uniform cavity temperature in order to prevent mode hops.
Finally, it is an object of the present invention to provide an optical parametric oscillator that is tunable over a wide temperature and frequency range without mode hops.
These objects and advantages are attained by a single-frequency optical resonator in which mode hops are prevented over a wide range of temperatures and frequencies by the inherent design of the resonator. The resonator has a composite cavity with a length chosen so that the peak gain frequency tuning rate matches the axial mode frequency tuning rate, thereby assuring that mode hops are prevented as the temperature shifts. In one embodiment, the resonator is solid state laser. The laser contains a solid state gain medium defining a physical gain path length Lg, a pump adjacent to the gain medium, a substrate, a high reflector, and an optical coupler. The high reflector and optical coupler are supported by the substrate and define a resonant cavity that surrounds the gain medium and has a round-trip physical cavity path length Lo.
The gain medium generates light of a characteristic frequency spectrum with a peak gain frequency "ugr"g and a peak gain frequency tuning rate with respect to temperature d"ugr"g/dT. It also has a refractive index ng, a thermo-optic coefficient dng/dT, and a thermal expansion coefficient (1/Lg) dLg/dT. Preferably, the gain medium generates waste heat at a rate of greater than 1 W; waste heat dissipation is confined to a region substantially near the gain medium. Suitable gain media include Nd:YAG, Nd:YVO4, Nd:YLiF4, Tm:YAG, Tm: YLiF4, Yb:YAG, Nd:glass, Ho: YLiF4, and Er:glass. Nd:YAG may be operated at both "ugr"g=2.818xc3x97105 GHz (i.e. xcex=1.064 xcexcm), in which case the peak gain frequency tuning rate d"ugr"g/dT=xe2x88x921.3 GHz/xc2x0 C., or at "ugr"g=2.273xc3x97105 GHz (xcex=1.319 xcexcm), with a peak gain frequency tuning rate d"ugr"g/dT=xe2x88x921.0 GHz/xc2x0 C.
The substrate is such that the cavity physical path length is temperature insensitive. Preferably, the substrate is thermally insensitive, having a negligible thermal expansion coefficient. For example, it may be Invar(trademark), Super-Invar(trademark), ULE(trademark) Glass, Zerodur(trademark), or fused silica. Alternately, it is thermally isolated: the substrate material is maintained at a constant temperature and is thermally insulated from the gain medium. The cavity supports lasing at a range of axial mode frequencies but operates at a specific axial mode frequency "ugr"m closest to "ugr"g. When Lo=Lg, the axial mode tuning rate has a greater magnitude than the peak gain frequency tuning rate, i.e, |d"ugr"m/dT| greater than |d"ugr"g/dT|. However, Lo/Lg is chosen so that the axial mode tuning rate d"ugr"m/dT is substantially equal to the peak gain frequency tuning rate d"ugr"g/dT. This ensures that axial mode frequency tuning occurs without axial mode hopping, preferably with the gain medium at a temperature between xe2x88x9220 xc2x0 C. and 80 xc2x0 C. In particular,             L      o              L      g        =      1    -          n      g        -                            ν          m                ⁡                  [                                                                      (                                                            n                      g                                        -                    1                                    )                                ⁢                                  1                                      L                    g                                                  ⁢                                                      ⅆ                                          L                      g                                                                            ⅆ                    T                                                              +                                                ⅆ                                      n                    g                                                                    ⅆ                  T                                                                                    ⅆ                                  ν                  g                                                            ⅆ                T                                              ]                    .      
For an Nd:YAG gain medium, Lo/Lg=2.11 at xcex=1.064 xcexcm and Lo/Lg=2.25 at xcex=1.319 xcexcm.
The laser preferably contains other standard elements for ensuring single frequency operation. Preferably, the laser is a ring oscillator, in which case it has means for ensuring unidirectional lasing, but it may also be a standing wave laser. For a ring oscillator, the gain medium is two cylindrical rods, each of which has two antiparallel Brewster face. The laser also contains a magnet adjacent to the gain medium to provide non-reciprocal polarization rotation. Reciprocal polarization rotation is provided by a half-wave plate inside the cavity, for a planar ring oscillator, or by the non-planar optical path of a non-planar ring oscillator. Preferably, the pumping means is two laser diode bars adjacent to the gain medium, each of which has a power output of at least 10 W, and the laser has a power output of at least 3 W.
In an alternate embodiment, the optical resonator is an optical parametric oscillator (OPO) and the gain medium is a nonlinear crystal. The same design is used to eliminate mode hops in the OPO.